Superheating of Steam
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On page 160 of his book “The Red Devil and Other Tales from the Age of Steam” Wardale confirms that:
“The fundamental laws of thermodynamics dictate that for the maximum thermal efficiency from any heat engine, the working fluid must commence its expansion from the highest possible temperature. In a steam locomotive this was accomplished by superheating the steam.”
He goes on to explain that superheating not only improves the ideal (Carnot) cycle efficiency but also gives other practical benefits not connected with the Second Law of Thermodynamics (and not often appreciated), such as:
Reduced heat transfer losses (condensation) to the cylinder walls,
Reduced steam flow pressure drop for a given volumetric flow rate, and
Reduced water consumption.
He goes on:
“Such was the benefit of the Schmidt type superheater realized in actual service that it must rank as the twentieth century’s most important single contribution to the art of steam locomotive design. Yet as with the exhaust system and feedwater heating, superheating was rarely fully exploited. The superheat temperature should have been the maximum possible – full stop – and this should have been all the motivation that was needed to improve the factors such as valve and cylinder lubrication which were said to be limiting it.
However all too often the reverse attitude seems to have been taken, these factors being seen as rigid barriers to higher temperatures and lower-than-possible ones being consequently accepted as the highest that could be allowed – or simply ‘good enough’. This was certainly the case on, for example, the SAR, where it was thought that steam temperatures of the order of 380°C as transitory values were all that were possible due to the lubrication issue. Worse still, the myth that high superheat merely wasted energy in the exhaust steam was believed by not a few engineers right down to recent times. On the other hand both Germany and France pursued means to allow the use of high steam temperatures: 400°C was normal in Germany on the standard designs first introduced in 1925 whilst Chapelon’s locomotives recorded temperatures as high as 425°C.”
Superheating is achieved by passing saturated steam from the “main pipe” through small diameter tubes called superheater elements which are placed inside large diameter boiler tubes called superheater “flues”. Combustion gases passing through these flues transmit their heat to the steam raising its temperature above (usually far above) its saturation value.
Superheater elements are connected to a superheater header – a small steam reservoir that is separated into two chambers. One end of each superheater element is connected to one of these chambers, and the other end to the second chamber. Saturated steam from the boiler enters the “saturated steam chamber” from where it passes through any one of the superheater elements and thence back into the second “superheated steam chamber” of the header. From there is passes through steam pipes to the steam chests and thence to the cylinders.
The steam regulator (or “throttle” as Wardale prefers to call it*) may be located on either side of the superheater header. In the case of the 5AT, the throttle is placed on the superheated steam side of the header.
[* Wardale prefers to use the term “throttle” because it better describes the action operating a locomotive with a partially-opened regulator.]
The picture below (of the sectioned Merchant Navy Pacific 35029 housed in the National Railway Museum in York) illustrates the arrangement of superheater flues, elements and header.
An alternative view, copied from a 1930s children’s book, shows the main steam pipe that delivers saturated steam to the superheater header. [Of interest is the steam collection pipe at the highest point of the domeless Belpaire (LMS) boiler.] Click on image to see full-size enlargement.
Note: In relation to the application of Feedwater Heating to “The Red Devil“, Wardale points out that “the need to increase the superheat on the modified 25NC was clear: not only was the existing steam temperature too low but the use of a feedwater heater always decreased the superheat and this factor had to be compensated for”.
This non-intuitive observation is explained in a footnote to a transcription of the text from the book which is summarized as follows: “Since less heat needs to be generated in the firebox to boil preheated water, then there’ll be less heat available for superheating the steam that is generated”.